Cold-core anticyclonic eddies south of the Bussol' Strait in the northwestern subarctic Pacific

Summer hydrographic surveys from 1993 to 1997 in the area south of the Kuril Islands in the northwestern subarctic Pacific showed the existence of anticyclonic eddies south of the Bussol' Strait at almost the same location but with variable sizes and intensities depending on the year. Every eddy had a cold, low salinity and low potential vorticity core, suggesting a strong influence from the Okhotsk Sea water. Two formation processes and annual variations were found with satellite data analyses. One is the case where eddies are locally formed south of the Bussol' Strait and intensified from summer to fall with the supply of Okhotsk Sea water as observed in 1993. In the other case, Kuroshio warm-core rings that had translated northeastward are arrested near the Bussol' Strait and amplified with the supply of Okhotsk Sea water as seen from summer to autumn in 1995. In winter, eddies tend to move northeastward with decay. The 1992 eddy moved northeastward then northward in winter and was eventually absorbed into the East Kamchatka Current. Mechanisms of the northeastward movements and the formations of the Bussol' eddies were discussed. A pseudo-β effect due to deep northeastward currents along the Kuril-Kamchatka Trench could be responsible for the northeastward movement. Since the volume transport of the coastal Oyashio water along the southern Kuril Islands is constrained by the potential vorticity difference between the Okhotsk Sea and the western subarctic gyre (WSAG), eddies could be generated and intensified when a outflow rate from the Okhotsk Sea exceeds the critical transport. The observed annual variations of the eddy evolution might be explained by the critical transport variation associated with an annual change of the WSAG.